I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to methods and apparatus for reducing multiple access interference in wireless communication systems that use code division multiple access techniques.
II. Description of the Related Art
Several multiple access communication techniques are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). However, the spread spectrum modulation techniques of code division multiple access (CDMA) provide significant advantages over other multiple access modulation. techniques for many applications. CDMA techniques in a communication system are disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention.
CDMA modulation techniques can provide capacity improvements over other techniques, such as TDMA and FDMA, based in part on CDMA""s use of orthogonal functions or codes. The CDMA codes are generated by, e.g., Walsh functions that mathematically form an orthogonal set. Thus, any two Walsh functions are orthogonal to each other, and signals encoded with two separate Walsh functions should cause no mutual interference when they are time aligned. An example of Walsh functions employed in a CDMA communication system is disclosed in U.S. Pat. No. 5,602,883, entitled xe2x80x9cMETHOD AND APPARATUS FOR USING WALSH SHIFT KEYING IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention. However, because multiple signals often are not time aligned, complete orthogonality is not achieved in practice. As a result, interference between otherwise orthogonal signals occurs. This is known as multiple access interference or MAI.
CDMA spreads the signal energy over a wide bandwidth. Therefore, fading of a CDMA signal is frequency selective. CDMA also provides space or path diversity through multiple signal paths that simultaneously link a mobile station or user with two or more cell-sites. Furthermore, CDMA can exploit the multipath environment by allowing a signal having multiple components each arriving at one receiver with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention.
Under one CDMA standard, described in the Telecommunications Industry Association""s TIA/EIA/IS-95-A Mobile Stations-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, each base station transmits pilot, sync, paging and forward traffic channels to its users. According to this standard, the pilot signal is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by each base station. The pilot signal allows each user to acquire information describing timing of signals within the channels transmitted by the base station, and provides a phase reference for coherent demodulation of these signals. The pilot channel also enables signal strength comparisons between base stations to determine when to hand off between base stations (such as when moving between cells).
CDMA modulation techniques require that all transmitters be under precise power control to in order to reduce interference between transmitted signals. When the power levels of signals transmitted by a base station to a user (the forward link) are too high, problems may be created, particularly MAI, as discussed above. As a result, most base stations have a fixed amount of power at which to transmit signals, and therefore can transmit to only a limited number of users. Alternatively, when the power levels of signals transmitted by the base station are too low, some users can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors also affect the power levels of signals transmitted by the base station. Thus, each base station needs to independently adjust the transmission power of the signals it transmits to each of its users. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention.
Even with independent power adjustment, CDMA communications systems are interference limited. As a result, any reduction in interference levels translates directly into increased system capacity. Conventional CDMA receivers utilize an approach of decoding a single signal. In this approach, all other user signals are considered to be noise. A more general approach that optimizes detection of all user signals simultaneously is too computationally intensive for real-time applications. As a result, considerable effort is presently being devoted to simpler algorithms for multi-user detection that provide improvement over the conventional CDMA receiver, but that are not as complex or as computationally intensive as the optimum approach. Examples of the considerations involved are discussed, for example, in xe2x80x9cMULTIUSER DETECTION FOR CDMA SYSTEMS,xe2x80x9d by A. Duel-Allen, J. Holtzman and Z. Zvonar (IEEE Personal Communications, Vol. 2, No. 2, pp. 46-58, April 1995) and xe2x80x9cMULTI-USER DETECTION FOR DS-CDMA COMMUNICATIONS,xe2x80x9d by S. Moshavi, (IEEE Communications Magazine, Vol. 34, No. 10, October 1996).
A number of these simpler algorithms have been developed. Some of these use an approach whereby a strongest signal from the received channel is selected. The selected signal is processed to recover data represented by the selected signal. The recovered data are re-processed to generate a replica of the strongest received signal. The replica is then subtracted from the signals in the received channel to provide a modified received signal. The modified received signal is then processed to recover the desired signal without interference that otherwise might have resulted from the strongest signal. One such approach is described in U.S. Pat. No. 5,719,852, entitled xe2x80x9cSPREAD SPECTRUM CDMA SUBTRACTIVE INTERFERENCE CANCELER SYSTEM.xe2x80x9d
A problem that may be encountered in such systems is that the replica of the strongest signal from the received channel may be in error. For example, if the sign of the recovered data is wrong, the subtraction will result in addition of the replica to the received signal, with the result that the MAI caused by mixing of the strongest signal with other signals in the received channel will be worse rather than better.
There are needs, therefore, for methods and apparatus for more effective CDMA receivers.
The inventors have found that pilot symbols or signals transmitted in CDMA systems may be usefully employed in estimating received signal strength. The estimate of received signal strength may then be used to estimate a probability that a strongest received signal has been correctly decoded. A reconstructed replica of the strongest received signal thereafter may be weighted by multiplying it with a value derived from the estimates, such as a non-linear value, to form a weighted replica of the strongest received signal. The weighted replica may be combined with the received signal to subtract contributions of the strongest received signal from the total received signal. The process then may be re-iterated to sequentially remove progressively weaker received signals until the desired signal is received and decoded without interference from the stronger signals. The accuracy with which the desired signal is decoded is enhanced because interference from stronger signals is reduced.
As a result, when there is a high probability of error in the estimate of the stronger received signals, a replica of the strongest signal is not subtracted from the received signals, and the desired signal is not further corrupted by erroneous corrections. However, when there is a low probability of error in the estimate of the stronger received signals, the desired signal is enhanced by successive subtraction of replicas of these stronger signals. In other words, if the estimated stronger received signal is likely erroneous, it is modified to affect the received signals by only a small amount, if at all, and if the estimate is likely to be correct, it is subtracted from the received signal in its full strength. This provides reduction in multiple access interference between CDMA signals that are simultaneously transmitted via a common channel.